Fisheye lens has experienced success in a number of applications involving panoramic or wide field-of-view applications. Such would include cinematography (U.S. Pat. No. 4,070,098), motionless surveillance (U.S. Pat. No. RE036,207), and image-based virtual reality (U.S. Pat. No. 5,960,108).
The advantage of fisheye projection is its large field of view compared to conventional rectilinear film. Images of field of view up to 220 degrees and beyond may be obtained with a fisheye lens. It has been speculated that a field of view infinitesimally less than 360 degrees is also obtainable, although the precise practical application of a lens of this type may be limited. In contrast, a conventional camera would require a rectilinear image recording surface of infinite dimensions for even 180 degrees of field of view.
As long ago as 1964, formal studies have been made of the optical characteristics of the fisheye lens (Kenro Miyamoto, “Fish eye lens”, Journal of Optical Society of America, 54:1060-1061, 1964). In 1983, Ned Greene suggested the use of fisheye images to generate environmental maps (Ned Greene, “A Method for Modeling Sky for Computer Animation”, Proc. First Int'l Conf. Engineering and Computer Graphics, pp.297-300, 1984).
In 1986, the use of perspective mapping for a fisheye image, projecting the latter into the sides of a rectangular box as an environmental map was introduced by Greene (Ned Greene, “Environmental Mapping and Other Applications of World Projections”, IEEE Computer Graphics and Applications, November 1986, vol. 6, no. 11, pp. 21-29). Greene took a 180 degree fisheye image (a fisheye environmental map) and projected it onto the six sides of a cube for perspective viewing.
Producing high-quality panoramic imaging using Greene's approach poses a number of difficulties. Each hemispheric image produces four half-sides of a cube, in addition to a full side, which require registration with its complement from the other hemispheric image. Registration of the half-images has two associated problems: spatial alignment and colour balancing.
Where the lens has a field of view greater than 180 degrees, the corresponding half-sides (as de-warped from the raw source image) require spatial alignment due to possible rotational and translational distortion. Furthermore, the image recording device would not necessarily capture the images precisely in the same area on the recording surface. Conventional methods of aligning the sides of such images are essentially manual in nature; even if assisted by graphics software, the process requires human intervention to line up the edges of half-sides based on the features in the images. This is often imprecise and difficult due to the multiple sources of distortion. There is a need for a more automatic process which reduces human intervention.
Chromatically, each half-side must be aligned relative to its complement. For example, the recording device may have the same exposure times and aperture openings for each of the two images despite the fact that the amount of light recorded for each image differs, as where the lighting changed between the two capture events, or alternatively, where the exposure times are different for each image despite equivalent lighting conditions. If aperture size is controlled automatically, further mismatches may result. As a result the complementary edges of half-images have different colour intensity but generally the same detail and level of noise.
Existing methodology for colour balancing tend to average the colour values of pixels in the relevant neighbourhood of the transition. This has the desired effect of bringing the difference across the transition into line. However, the disadvantage concerns the concomitant loss of detail. As a result there is a perceptible blurring across the region of the transition. When the width of the overlapped region is narrow as compared to lighting imbalance, the transition may appear too abrupt.